1. Field of the Invention
The present invention relates to a position control apparatus that can be applied to a numerically controlled machine.
2. Related Art
Conventionally, a machine tool or a similar industrial machine is equipped with a feed-axis driving mechanism that includes a ball screw. For example, the machine tool includes a servomotor that can rotate the ball screw fixed to an output shaft thereof and is configured to linearly drive a load fixed to a ball screw nut. However, according to the above-described mechanism, a ball screw portion may cause deflection due to the weight of the load.
FIG. 4 is a block diagram illustrating an example of a conventional position control apparatus. A position command value y* generated by a host apparatus (not illustrated in the drawing) is converted into a motor rotation command value θ*. The position control apparatus receives the motor rotation command value θ* as an input command.
The conventional position control apparatus employs a feed-forward control system to improve the response of the motor. The feed-forward control system can compensate for deflection that may be generated by the ball screw. A deflection amount θdf of the ball screw can be obtained according to the following formula 1, when Mh represents a load weight, Ks represents a rigidity of a ball screw driving system, and θ* represents a position command value.θdf=(Mh/Ks)×(d2θ*/dt2)   formula 1
A block Ca (=(Mh/Ks)×s2) can calculate the deflection amount θdf according to the above-described formula 1. An adder 51 can add the position command value θ* to the deflection amount θdf to generate a position command value θc for the control. The control system can time-differentiate the deflection amount θdf to obtain a deflection speed Vdf. The control system can further time-differentiate the position command value θ*. An adder 53 can add the deflection speed Vdf to the time-differential value of the position command value θ* to generate a speed feed-forward amount Vff. The control system can further time-differentiate the speed feed-forward amount Vff to obtain an acceleration feed-forward amount Aff. A conversion block Cb can obtain a torque feed-forward amount τff based on the acceleration feed-forward amount Aff. The torque feed-forward amount τff corresponds to a motor torque that can generate acceleration equivalent to the acceleration feed-forward amount Aff.
The conventional position control apparatus further includes the following feedback control configuration. An objective plant 50 to be controlled by the position control apparatus has a mechanism (not illustrated) that includes a servo motor configured to drive a load via a ball screw based on a torque command value τm that is input as a manipulated variable. The servo motor includes a position detector (not illustrated) incorporated therein, which can detect a motor rotational angle θm and an angular speed vm. A linear scale (not illustrated), which is provided separately, detects a position “y” of the load. The control system multiplies the load position “y” of the load by a constant (2π/P) to obtain a load position θi as a conversion value on a motor rotational axis, considering a lead P of the ball screw.
A subtracter 52 can subtract a position feedback value θf from the position command value θc for the control. A position deviation amplifier Gp can amplify a position deviation (i.e., an output of the subtracter 52). An adder 54 can add the output of the position deviation amplifier Gp to the speed feed-forward amount Vff to generate a speed command value Vc. A subtracter 55 can subtract the motor rotation angular speed vm from the speed command value Vc to obtain a speed deviation. A speed deviation amplifier Gv can perform proportional-integral amplification on the speed deviation (i.e., the output of the subtracter 55). An adder 56 can add the output of the speed deviation amplifier Gv to the torque feed-forward amount τff to generate the torque command value (i.e., motor generation torque) τm.
The following formula 2 represents a relationship of the position feedback value θf and other parameters θm, θdf, and θi, which can be obtained referring to FIG. 4.θf=θm+G(S)[θi−(θm−θdf)]  formula 2
In the formula 2, G(S) has delay system transfer characteristics, according to which G(S) takes a larger value for a low-frequency input and a smaller value for a high-frequency input in a range of 0≦|G(S)|≦1.
Accordingly, in a stable condition, a relationship θf=θi+θdf can be satisfied. The feedback control can equalize the position command value θc with the position feedback value (i.e., θc=θf). Therefore, a relationship θ*=θi can be attained. In short, the control system can control the load position “y” accurately according to the position command value θ*. The above-described conventional position control apparatus is discussed, for example, in JP 3351990 B.
As described above, the conventional position control apparatus performs control for equalizing the position command value θ* with the load position θi by advancing the motor position (i.e., motor rotational angle) θm by an amount corresponding to the deflection amount θdf caused in the ball screw portion. However, a structural member that supports and fixes the driving system may have a low rigidity. An element having a lower rigidity may be present beyond a load position where the detection by the linear scale is performed. In such cases, the above-described conventional position control apparatus cannot compensate deflection components caused by machine elements at the lower-rigidity portions. Furthermore, in a machine having a portion that is weak or insufficient in mechanical rigidity, if it is required to drive the feed-axis with high-acceleration or deceleration, a deflection amount of a mechanical system increases with increasing acceleration.
The deflection in the mechanical system is described below in more detail. FIG. 2A illustrates a model representing a schematic mechanism of a double-column machining center as an example of a machine tool, which is one of the numerically controlled machines. Abed 11 is stationarily fixed on the ground. A table 12, which is disposed on the bed 11, can move in the X direction. A workpiece 18, i.e., an object to be machined or processed, is mounted and fixed on the table 12.
Similar to the bed 11, a pair of columns 13 is rigidly fixed on the ground. A cross rail 14 can move relative to the columns 13 in a direction indicated by W. A saddle 15 can move relative to the cross rail 14 in a direction indicated by Y. The double-column machining center includes a mechanism capable of moving a ram 16 in a direction indicated by Z (i.e., in the up-and-down direction). The ram 16 has a front end equipped with a spindle head. The double-column machining center can rotate a tool 17 attached to the spindle head of the ram 16 at a higher speed to cut (process) the workpiece 18.
Hereinafter, an operational movement in the Y direction is described below. An example system can control a servomotor (not illustrated), which is installed on the cross rail 14 and serves as a Y-axis driving system, to drive the saddle 15 via a ball screw (not illustrated). A linear scale (not illustrated) attached to the cross rail 14 can detect the position of the saddle 15.
The conventional position control apparatus can compensate a deflection component generated in the ball screw portion to equalize the position command value θ* with the position θi of the saddle 15. However, the columns 13 supporting the cross rail 14 may have a lower rigidity, or the degree of coupling between the columns 13 and the ground may be low. In such cases, for example, if the saddle 15 accelerates in the −Y direction, the cross rail 14 and the columns 13 receive reaction forces and displace in the +Y direction, as illustrated in FIG. 2B. In this case, even when the position command value θ* is equal to the position θi of the saddle 15 detected by the linear scale, the absolute position of the saddle 15 in the space deviates from a desired position by a displacement amount of the columns 13.
Similarly, when the rigidity of the ram 16 is low, if the saddle 15 accelerates in the −Y direction, the spindle head attached to the front end of the ram 16 receives an inertia force. As a result, the spindle head displaces in the +Y direction. Accordingly, a front end of the tool 17 deviates from a desired locus in the space. Therefore, the tool 17 causes a displacement relative to the workpiece 18 (i.e., an object to be processed), and the tool 17 cannot perform a cutting operation at an appropriate position.
Furthermore, the load acting point changes depending on the position of the cross rail 14 relative to the columns 13 (i.e., a W-axis coordinate value) or depending on a protruding amount (i.e., a Z-axis coordinate value) of the ram 16 relative to the saddle 15. In other words, the mechanical rigidity of the columns 13 or the ram 16 changes and, as a result, the amount of a deflection itself to be generated when the saddle 15 accelerates is variable.
The problems to be solved by the present invention include a phenomenon that a deflection may be generated in a mechanical system due to a structural member having a lower rigidity that supports and fixes a driving system, and the locus of a front end of a tool may cause a displacement relative to a workpiece to be processed. Furthermore, the displacement amount changes according to a machine posture.
An object of the present invention is to provide a position control apparatus that can constantly compensate a deflection that may be generated in the mechanical system, even if the machine posture changes. Thus, the position control apparatus according to the present invention can move the front end of the tool along a desired locus.